Automatic focusing device of image pickup device and method of correcting zoom tracking curve

ABSTRACT

The present invention is aimed to provide an autofocus device and a correction method of correcting a zoom tracking curve for an intermediate subject distance without increase in storage capacity. A plurality of zoom tracking curves L1(Z) and L2(Z) corresponding to a plurality of different subject distance and correction data α(Z) calculated in advance are stored in a memory  16 . A control circuit  10  carries out a predetermined interpolation process using the stored plurality of zoom tracking curves and correction data to calculate a zoom tracking curve L(Z) corrected with respect to a subject distance differing from the plurality of subject distance. Tracking control of focus is carried out according to this calculated zoom tracking curve.

TECHNICAL FIELD

The present invention relates to autofocus devices, and moreparticularly, to an autofocus device including a software improved forfocus tracking control of high accuracy in image sensing apparatus suchas a video camera.

BACKGROUND ART

In the autofocus control of image sensing apparatus such as a videocamera, focus tracking control is carried out according to each zoomposition on the basis of a zoom tracking curve indicating therelationship between the zoom position of a group of zoom lens and thefocus position of a group of focus lens. A table of the data of the zoomtracking curve is stored in the memory of the autofocus device.

The zoom tracking curve differs according to the distance to a subjectof interest. If tracking control of focus in high accuracy is to becarried out with respect to every subject distance, a table includingall zoom tracking curves corresponding to every subject distance must beprovided. Therefore, a great amount of storage capacity for data will berequired.

Accordingly, only a table storing several typical zoom tracking curves,i.e., zoom tracking curves for the cases where the distance up to asubject corresponds to an infinity point and to a nearby point isprovided, and zoom tracking curves corresponding to other subjectdistance are obtained by interpolation processing that geometricallydivides between such prestored zoom tracking curves.

The calculated value of a zoom tracking curve obtained from theabove-described interpolation processing by geometric division of a fewnumber of zoom tracking curves substantially matches the design valueobtained definitely for each subject distance in the design stage as faras the axis of lens and the proximity thereof is concerned.

However, in an area distant in the radial direction from the axis oflens, the error between the calculated value by interpolation processingand the design value obtained by actual designing will become greaterdue to spherical aberration of lens to result in a noticeable out-offocus state.

Such an error is particularly significant with respect to a subjectdistance that is around the middle of respective subject distancecorresponding to the plurality of, for example 2, prestored zoomtracking curves used as the basis for interpolation processing. Thetrend of a noticeable defocused state is noted.

For the purpose of reducing such an error between a calculated value anda design value to improve the accuracy of focus tracking, a table with agreater number of zoom tracking curves must be provided. However, thisinduces the problem of increase in the data storage capacity andcomplication in data processing.

Therefore, an object of the present invention is to provide an autofocusdevice that has the focus tracking accuracy improved significantlywithout having to greatly increase the data storage capacity.

DISCLOSURE OF THE INVENTION

According to the invention defined in claim 1, an autofocus device ofimage sensing apparatus includes memory means for storing data of aplurality of zoom tracking curves corresponding respectively to aplurality of different subject distance and precalculated correctiondata, calculation means carrying out a predetermined interpolationprocess using said plurality of zoom tracking curves and said correctiondata for calculating a zoom tracking curve with respect to a subjectdistance differing from said plurality of subject distance, and trackingmeans for tracking focus according to a calculated zoom tracking curve.

According to the invention defined in claim 2, an autofocus device ofimage sensing apparatus includes memory means for storing data of aplurality of zoom tracking curves corresponding respectively to aplurality of different subject distance and correction data calculatedaccording to a zoom tracking curve obtained by a predeterminedinterpolation process with respect to a predetermined subject distancethat is intermediate said plurality of subject distance, calculationmeans carrying out a predetermined interpolation process using said dataof a plurality of zoom tracking curves and said correction data forcalculating a zoom tracking curve with respect to a subject distancediffering from said plurality of subject distance, and tracking meansfor tracking focus according to said calculated zoom tracking curve.

According to the invention defined in claim 3, said calculation means inthe autofocus device of claim 2 includes means for calculating a focusposition which is said correction data subtracted from a current focusedposition, means for geometric-dividing said plurality of zoom trackingcurves on the basis of said calculated focus position, and means foradding said correction data to said zoom tracking curve obtained bygeometric division for obtaining a zoom tracking curve with respect tosaid different subject distance.

According to the invention defined in claim 4, said correction data iscalculated according to the error between said zoom tracking curveobtained with respect to said intermediate predetermined subjectdistance and the design value in the autofocus device of claims 1-3.

According to the invention defined in claim 5, a method of correcting azoom tracking curve in an autofocus device includes the steps of:preparing memory means in which are stored data of a plurality of zoomtracking curves respectively corresponding to a plurality of differentsubject distance and correction data calculated according to a zoomtracking curve obtained by a predetermined interpolation process withrespect to a predetermined subject distance intermediate said pluralityof subject distance, calculating a focus position which is saidcorrection data subtracted from a current focused position,geometric-dividing said plurality of zoom tracking curves on the basisof said calculated focus position, and adding said correction data tosaid curve obtained by geometric division for obtaining a zoom trackingcurve for a subject distance differing from said plurality of subjectdistance.

According to the invention defined in claim 6, said correction data iscalculated according to the error between the zoom tracking curveobtained for said intermediate predetermined subject distance and adesign value in the correction method of claim 5.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic block diagram showing a structure of a videocamera according to an embodiment of the present invention;

FIG. 2 is a diagram for describing the method of basic geometricdivision of a zoom tracking curve;

FIG. 3 is a diagram showing the error between an interpolated zoomtracking curve and a design value; and

FIG. 4 is a flow chart showing an interpolation process according to anembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a schematic block diagram showing a structure of a videocamera according to an embodiment of the present invention.

In FIG. 1, a lens unit 1 includes a front lens group 1 a, a zoom lensgroup 1 b for effecting variable magnification function, an elector lensgroup 1 c, and a rear (focus) lens group 1 d for effecting thein-focusing function.

Light from a subject not shown passes through each of the lens group inlens unit 1 to form an image on the image sensing plane of an imagesensing element 2, and then converted into an electrical signal. A videosignal processing circuit 3 carries out a predetermined process on theelectrical signal received from image sensing element 2 for conversioninto a video signal of the NTSC or PAL system.

The video signal is recorded on a recording medium by a recordingcircuit not shown or output outside as the pickup output of a videocamera, and simultaneously has the high frequency component extractedvia a high pass filter 4 to be applied to an analog/digital converter 5.

The high frequency component of the video signal that is converted intoa digital signal by analog/digital converter 5 is applied to anautofocus circuit 6 as a focus evaluation value by the well-knownhill-climbing autofocus control method. Autofocus circuit 6 generatesand provides to a control circuit 10 the focus information to providefocus control on focus lens group 1 d according to the focus evaluationvalue. Control circuit 10 provides the overall control of the operationof the video camera.

Zoom lens group 1 b is driven by a zoom motor 7 under control of controlcircuit 10. The position thereof (zoom position) is detected by anencoder 8 formed of a slide volume. The detected output of encoder 8 isconverted into a digital signal by an analog/digital converter 9 to beinput to control circuit 10.

A pulse motor can be used as zoom motor 7. In this case, encoder 8 isnot required. Control circuit 10 will discern the zoom positionaccording to the number of driven pulses output from the pulse motor.

Focus lens group id is driven by a pulse motor 11 under control ofcontrol circuit 10. A reference position sensor 12 generates andprovides to control circuit 10 a detection signal when focus lens group1 d is located at a preset reference position.

Since pulse motor 11 rotates in proportion to the number of drivenpulses input from control circuit 10, control circuit 10 identifies theposition (focus position) of focus lens group id according to the outputof reference position sensor 12 and the number of driven pulses outputtowards pulse motor 11.

Control circuit 10 is further connected with a zoom switch 13, a focusmode switch 14, and a manual focus switch 15.

The user operates zoom switch 13 to drive zoom motor 7, whereby the zoomposition is altered.

When the user operates focus mode switch 14 to select an autofocus mode,control circuit 10 drives pulse motor 11 according to the focusinformation signal from autofocus circuit 6, whereby focus lens group 1d is moved so that the focus evaluation value is maximum.

When the user operates focus mode switch 14 to select a manual focusmode, pulse motor 11 is driven upon operation of manual focus switch 15by the user. As a result, the focus position is altered.

Control circuit 10 is yet further connected with a nonvolatile memory 16in which various distance information and position information of thedata for a zoom tracking curve are stored, and a power supply switch 17.A power supply circuit 18 serves to supply power to each element in thevideo camera. Control circuit 10 controls the operation of power supplycircuit 18 according to manipulation of power supply switch 17 by theuser.

In an autofocus operation of the video camera shown in FIG. 1, controlcircuit 10 calculates the focus position (focused position) of focuslens group 1 d corresponding to the position (zoom position) of zoomlens group 1 b detected by encoder 8, according to the zoom trackingcurve obtained by an interpolation process of the plurality of zoomtracking curves prestored in memory 16. Focus lens group 1 d is moved tothe calculated focus position.

The basic interpolation method employing geometric division of a zoomtracking curve will be described with reference to FIG. 2. FIG. 2 is agraph showing a zoom tracking curve in which the zoom position isplotted along the abscissa and the focus position is plotted along theordinate.

Here, an example will be described of obtaining a zoom tracking curveL(Z) with respect to an intermediate subject distance L through aninterpolation process by geometric division between two zoom trackingcurves L1(Z) and L2(Z) (Z is an arbitrary zoom position) with respect totwo different subject distance L1 and L2 out of the plurality of zoomtracking curves stored in memory 16.

First, values L1(T) and L2(T) taken by the two zoom tracking curvesL1(Z) and L2(Z) at one end position T of the telephoto side of the zoomposition along the abscissa are divided by a certain ratio. A new zoomtracking curve L(Z) is drawn that constantly takes a value L(Z). L(Z)corresponds to the values taken by zoom tracking curves L1(Z) and L2(Z)at an arbitrary zoom position Z divided by the same certain ratio.

More specifically, when the value of L1(T) minus L(T) is a(T) and thevalue of L(T) minus L2(T) is b(T) at the end portion T of the zoomposition, and the value of L1(Z) minus L(Z) is a(Z)=L1(Z)−L(Z) and thevalue of L(Z) minus L2(Z) is b(Z)=L(Z)−L2(Z) at the arbitrary zoomposition Z, a new zoom tracking curve with respect to subject distance Lby geometric division is obtained by drawing a curve L(Z) that alwayssatisfies a (Z):b (Z)=a (T):b (T).

However, such a zoom tracking curve obtained by an interpolation processof geometric division may cause defocus, as described before, since itincludes an error at the subject distance in the proximity of the areaintermediate the subject distance of the two tracking curves. In view ofthe foregoing, the present invention carries out error correction in theinterpolation process by the method set forth in the following.

Considering that the error is greatest with respect to the mostintermediate subject distance of respective subject distance L1 and L2for zoom tracking curves L1(Z) and L2(Z), a zoom tracking curve for themost intermediate subject distance is first drawn by the above-describedgeometric division. Then, the error between a calculated value on theobtained curve and a corresponding design value is identified at eachsubdivided point of the zoom position that is equally divided.

FIG. 3 shows a curve of the design value (dotted line) with respect tothe zoom tracking curve L obtained by interpolation processing, and anerror δ therebetween. It is appreciated from FIG. 3 that this error δ isa function δ(Z) that changes depending upon zoom position Z.

As a result of actually confirming and evaluating the tracking accuracyand the like, correction value α(Z) with respect to the calculated valueat each zoom position is determined as α(Z)=δ(Z)/2. This correction dataα(Z) is prestored in memory 16.

A method of the interpolation process on a zoom tracking curve employingsuch correction data α(Z) will be described specifically according tothe present invention hereinafter.

It is assumed that an in-focus state is obtained at a certain zoomposition (for example, the position corresponding to end portion T inFIG. 2) with respect to a subject at a subject distance betweenreferenced zoom tracking curves L1(Z) and L2(Z). When tracking is to becommenced from that position, correction value α=δ/2 corresponding tothat zoom position is read out from the memory. A focus position iscalculated by subtracting this correction value α from the focusedposition.

Then, the aforementioned geometric division is carried out on two knownzoom tracking curves corresponding to that zoom position according tothe focus position obtained by subtracting correction value α to obtainan interpolated zoom tracking curve. A curve having correction valueα(Z) added to that obtained curve is taken as the eventual correctedzoom tracking curve. Focus tracking control is carried out according tothis curve.

In the embodiment of the present invention of FIG. 1, such aninterpolation process is executed using a software by control circuit 10formed of a microcomputer. Such a software will be described withreference to FIG. 4. In the following description, the zoom trackingcurve corresponding to a nearby point L1 is L1(Z), and the zoom trackingcurve corresponding to a remote point L2 is L2(Z), as to the twodifferent subject distance of L1 and L2.

At step S1, control circuit 10 identifies the current zoom position Z′and the focused position k according to the outputs of encoder 8 andreference position sensor 12, assuming that an in-focus state isobtained at the current zoom position Z′.

At step S2, control circuit 10 identifies correction value αcorresponding to the current zoom position according to the data of precalculated correction value α(Z) stored in memory 16.

At step S3, control circuit 10 calculates focus position k−α that is thecurrent identified focused position k minus correction value α.

At step S4, control circuit 10 identifies focused positions L1(Z′) andL2(Z′) for the current zoom position according to the two zoom trackingcurves L1(Z) and L2(Z) prestored in memory 16 to enter the geometricdivision process.

At step S5, control circuit 10 calculates value “a” that is L1(Z′) minus(k−α) and value “b” that is (k−α) minus L2(Z′) (corresponding to a (T),b(T) of FIG. 2).

At step S6, a tracking curve L′(Z) is drawn so that the relationshipL1(Z):L2(Z)=a:b is always established at arbitrary zoom position Z.

At step S7, a zoom tracking curve L(Z)=L′(Z)+α(Z) exceeding trackingcurve L′(Z) by correction value α(Z) read out from memory 16 iscalculated. At step S8, focus tracking control is carried out accordingto zoom tracking curve L(Z).

INDUSTRIAL APPLICABILITY

According to the present invention, correction based on an error in aconventional interpolation process is applied in the interpolationprocess of a zoom tracking curve. Therefore, defocus will not occur inthe focus tracking control with respect to a subject located at anintermediate subject distance. Only the storage capacity for thecorrection data is to be added. Thus, focus tracking control of highaccuracy can be realized by a slight addition of storage capacity incontrast to the case where the data of zoom tracking curves for everysubject distance is stored.

What is claimed is:
 1. An autofocus device of image sensing apparatus,comprising: a memory to store data of a plurality of zoom trackingcurves corresponding respectively to a plurality of different subjectdistances, and correction data calculated in advance; a control circuitto carry out a predetermined interpolation process using said data of aplurality of zoom tracking curves and said correction data forcalculating a zoom tracking curve with respect to a subject distancediffering from said plurality subject distances, said correction datacalculated based on an error function in the calculated zoom trackingcurve, and to track focus according to said calculated zoom trackingcurve, wherein said correction data is calculated based on the formulaα(Z)=δ(Z)/2, where α is the correction value, Z is the zoom position,and δ is an error value between the calculated zoom tracking curve and adesign curve.
 2. A method of correcting a zoom tracking curve of anautofocus device, said method comprising the steps of: preparing memoryin which are stored data of a plurality of zoom tracking curvescorresponding respectively to a plurality of different subject distance,and correction data calculated according to a zoom tracking curveobtained by a predetermined interpolation process with respect to apredetermined subject distance that is intermediate said plurality ofsubject distance, calculating a focus position which is said correctiondata subtracted from a current focus position, geometric-dividing saidplurality of zoom tracking curves on the basis of said calculated focusposition, calculating, in advance of preparing said memory means, saidcorrection data based on the formula α(Z)=δ(Z)/2, where α is thecorrection value, Z is the zoom position, and δ is an error valuebetween the calculated zoom tracking curve and a design curve; andadding said correction data to said curve obtained by geometric divisionto obtain a zoom tracking curve with respect to a subject distancediffering from said plurality of subject distances.
 3. The zoom trackingcurve correction method according to claim 2, wherein said correctiondata is calculated according to said zoom trackingcurve obtained withrespect to said intermediate predetermined subject distance and a designvalue.
 4. An autofocus device of image sensing apparatus, comprising: amemory to store data of a plurality of zoom tracking curvescorresponding respectively to a plurality of different subjectdistances, and correction data calculated according to a zoom trackingcurve obtained by a predetermined interpolation process with respect toa predetermined subject distance that is intermediate said plurality ofsubject distance; a control circuit to carry out a predeterminedinterpolation process using said data of a plurality of zoom trackingcurves and said correction data for calculating a zoom tracking curvewith respect to a subject distance differing from said plurality ofsubject distances, said correction data calculated based on an errorfunction in the calculated zoom tracking curve, and to track focusaccording to said calculated zoom tracking curve, wherein saidcorrection data is calculated based on the formula α(Z)=δ(Z)/2, where αis the correction value, Z is the zoom position, and δ is an error valuebetween the calculated zoom tracking curve and a design curve.
 5. Theautofocus device of image sensing apparatus according to claim 4,wherein said control circuit calculates a focus position which is saidcorrection data subtracted from a current focused position,geometrically-divides said plurality of zoom tracking curves on thebasis of said calculated focus position, and adds said correction datato said curve obtained by geometric division for obtaining a zoomtracking curve with respect to said different subject distance.
 6. Theautofocus device of image sensing apparatus according to any one ofclaims 5, 3, or 1, wherein said correction data is calculated accordingto a zoom tracking curve obtained with respect to said intermediatepredetermined subject distance and a design value.